Closed by MichaelGiacomelli (saratoga)
Thursday, 28 March 2013, 22:04 GMT Reason for closing: Out of DateAdditional comments about closing: As far as we can tell, enabling this has
almost no effect on actual performance
driving low impedance headphones
(measured THD improved slightly but the
difference is so small I'm not sure
its real) but it does seem to reduce
battery life by a fair amount (45 minute
reduction was reported in one test). If
someone does a new test showing a
difference or that battery life is no
longer negatively affected, we can
reopen this but for now this patch seems
like it should not be used.

This did not improve playback on the clipv2 for me. The tone I hear doesn't respond to volume changes, so changing the HPH settings is unlikely to have any effect on it. Increasing the bias current will probably reduce things like THD at high volume though.

To clarify, this only has to do with nonlinear distortion levels in mixer and headphone amplifier. The AS3543 datasheet states the increased bias current should be helpful for loads of *less* than 32 ohm in particular, and I doubt they were going by audiophile standards when they wrote that. The distortion spectrum with a low-impedance load certainly suggests an underbiased output stage.

True, it would be useful to have an option to turn this on and off dynamically. "On" would be "audiophile mode" and "off" would be "power saving mode".

I'd be fine enabling this unconditionally. I believe this also would be an easier sell than an option for "audiophile" playback: I think the Rockbox project as a whole values playback quality slightly higher than extended battery life, whereas there's always push-back against adding yet another option.

Small improvement in THD leading to a grand total of 0.1 dB improvement in (already extremely low) noise floor. Unless there's literally no change in battery life I don't really see a point in having this. Such small changes in SNR are barely measurable, and certainly not audible. The AGC change in FS#12111 is probably worth having though.